Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

Frustrated Kondo impurity triad: A toy model of deconfinement

Elio J. König, Piers Coleman, Yashar Komijani

Published Feb 27, 2020• 3 citations
OpenAlex

Abstract

The concepts of deconfinement and topological order are of great current interest for quantum information science and for our understanding of quantum materials. Here, we introduce a simple model of three antiferromagnetically coupled Kondo impurities, a ``Kondo triangle'', which can be used to further extend the application of these concepts to electronic systems. We show that by tuning the magnetic frustration, the Kondo triangle undergoes a quantum phase transition between two phases of unbroken symmetry, signaling a phase transition beyond the Landau paradigm. We demonstrate that the frustrated ``spin liquid'' phase is described by a three-channel Kondo fixed point and thus displays an irrational ground state degeneracy. Using an Abrikosov pseudofermion representation this quantum state is categorized by an emergent U(1) gauge field and its projective symmetry group. The gauge theory is deconfining in the sense that a miniature Wilson loop orders and that topological defects (instantons in the gauge field) are expelled. This phase persists in presence of moderate Kondo screening until the proliferation of topological defects leads to a quantum phase transition to an unfrustrated Fermi liquid phase. Based on this evidence, we propose that the three-channel Kondo phase displays topological order in a similar sense as gapless spin liquids.

Related papers

Frustrated Kondo impurity triangle: A simple model of deconfinement

Elio J. König, Piers Coleman, Yashar Komijani

Concepts of topological order are central to our understanding of strongly correlated quantum materials. In this work, the authors propose that such concepts -- which transcend the Landau paradigm -- also appear in frustrated impurity models. In particular, they show that a variety of defining characteristics of topological order appear in a frustrated Kondo triangle: an irrational ground-state degeneracy reminiscent of anyons, ordering of a Wilson-loop-like order parameter, and a phase transition driven by the proliferation of monopoles of an emergent gauge field.

arXiv:2002.12338•Physical review. B./Physical review. B•Sep 3, 2021•9 citations

Topological ground state degeneracy of the two-channel Kondo lattice

Aleksandar Ljepoja, Yashar Komijani

There has been significant recent interest in the potential exotic properties of multichannel Kondo lattices and their connection to quantum materials, but the strongly coupled nature of these models has posed challenges to theoretical investigations. We study the emergence of topological order in a two-dimensional two-channel Kondo lattice (2CKL) in a time-reversal symmetry-broken channel symmetric phase using a coupled-wire construction from one-dimensional (1D) 2CKLs. Through bosonization and symmetry analysis, we show that away from particle-hole symmetry, the 1D 2CKL model flows to an infrared fixed point with an emergent $\mathrm{SO}(3,1)\ifmmode\times\else\texttimes\fi{}\mathrm{SU}(2)$ symmetry and a ${\mathbb{Z}}_{2}$ gauge structure. Interwire couplings induce a transition to a topologically ordered metallic state characterized by fractionalized excitations and eightfold ground state degeneracy on a torus. Our findings connect impurity-based non-Fermi-liquid behavior to lattice topological order, with potential implications for quantum materials and topological quantum computation.

arXiv:2410.01759•Physical review. B./Physical review. B•Nov 10, 2025•2 citations

Kondo Impurities in the Kitaev Spin Liquid: Numerical Renormalization Group Solution and Gauge-Flux-Driven Screening

Matthias Vojta C, Andrew K. Mitchell, Fabian Zschocke

Kitaev's honeycomb-lattice compass model describes a spin liquid with emergent fractionalized excitations. Here, we study the physics of isolated magnetic impurities coupled to the Kitaev spin-liquid host. We reformulate this Kondo-type problem in terms of a many-state quantum impurity coupled to a multichannel bath of Majorana fermions and present the numerically exact solution using Wilson's numerical renormalization group technique. Quantum phase transitions occur as a function of Kondo coupling and locally applied field. At zero field, the impurity moment is partially screened only when it binds an emergent gauge flux, while otherwise it becomes free at low temperatures. We show how Majorana degrees of freedom determine the fixed-point properties, make contact with Kondo screening in pseudogap Fermi systems, and discuss effects away from the dilute limit.

arXiv:1604.03557•Physical Review Letters•Jul 12, 2016•37 citations

Boundary quantum criticality in models of magnetic impurities coupled to bosonic baths

Serge Florens, Lars Fritz, Matthias Vojta C

We investigate quantum impurity problems, where a local magnetic moment is coupled to the spin density of a bosonic environment, leading to bosonic versions of the standard Kondo and Anderson impurity models. In a physical situation, these bosonic environments can correspond either to deconfined spinons in certain classes of ${Z}_{2}$ frustrated antiferromagnets, or to particles in a multicomponent Bose gase (in which case the spin degree of freedom is attributed to hyperfine levels). Using renormalization group techniques, we establish that our impurity models, which feature an exchange interaction analogous to Kondo impurities in Fermi liquids, allow flow toward a stable strong-coupling state. Since the low-energy bosons exist around a single point in momentum space, and there is no Fermi surface, an impurity quantum phase transition occurs at intermediate coupling, separating screened and unscreened phases. This behavior is qualitatively different from previously studied spin-isotropic variants of the spin-boson model, which display stable intermediate-coupling fixed points and no screening.

arXiv:cond-mat/0703609•Physical Review B•Jun 15, 2007•9 citations

Order parameter for the multichannel Kondo model at quantum criticality

Ru Zheng, Rong-Qiang He, Zhong-Yi Lu

A multichannel Kondo model, where two or more equivalent but independent channels of electrons compete to screen a spin-$\frac{1}{2}$ impurity, shows overcompensation of the impurity spin, leading to the non-Fermi-liquid behavior in various thermodynamic and transport properties. However, when the channel symmetry is broken, an impurity quantum phase transition can occur at zero temperature. Identification of an order parameter describing the impurity quantum phase transition is very difficult since it is beyond the conventional Landau-Ginzburg-Wilson theory. By employing the natural orbitals renormalization group method, we study both two-channel and three-channel Kondo models, from the perspective of spin correlation between the impurity and electrons in electronic channels. Here we demonstrate that by introducing the spin-correlation ratio as an order parameter we can characterize impurity quantum phase transitions driven by channel asymmetry. In particular, the universal critical exponents $\ensuremath{\beta}$ of the spin-correlation ratio and $\ensuremath{\nu}$ of the correlation length are explicitly determined by finite-size-scaling analysis, namely, $\ensuremath{\beta}=0.10(1)$ and $\ensuremath{\nu}=2.0(1)$, and $\ensuremath{\beta}=0.10(1)$ and $\ensuremath{\nu}=2.5(1)$ for the two-channel and three-channel Kondo models, respectively.

arXiv:2201.04303•Physical review. B./Physical review. B•Jan 11, 2021•5 citations

Two-channel pseudogap Kondo and Anderson models: Quantum phase transitions and non-Fermi liquids

Imke Schneider, Lars Fritz, Frithjof B. Anders +2 more

We discuss the two-channel Kondo problem with a pseudogap density of states $\ensuremath{\rho}(\ensuremath{\omega})\ensuremath{\propto}{|\ensuremath{\omega}|}^{r}$ of the bath fermions. Combining both analytical and numerical renormalization group techniques, we characterize the impurity phases and quantum phase transitions of the relevant Kondo and Anderson models. The line of stable points, corresponding to the overscreened non-Fermi-liquid behavior of the metallic $r=0$ case, is replaced by a stable particle-hole-symmetric intermediate-coupling fixed point for $0<r<{r}_{\mathrm{max}}\ensuremath{\approx}0.23$. For $r>{r}_{\mathrm{max}}$, this non-Fermi-liquid phase disappears, and instead a critical fixed point with an emergent spin-channel symmetry appears, controlling the quantum phase transition between two phases with stable spin and channel moments, respectively. We propose low-energy field theories to describe the quantum phase transitions, all being formulated in fermionic variables. We employ $\ensuremath{\epsilon}$ expansion techniques to calculate critical properties near the critical dimensions $r=0$ and 1, the latter being potentially relevant for two-channel Kondo impurities in neutral graphene. We find the analytical results to be in excellent agreement with those obtained from applying Wilson's numerical renormalization group technique.

arXiv:1108.2213•Physical Review B•Sep 29, 2011•18 citations
The concepts of deconfinement and topological order are of great current interest for quantum information science and for our understanding of quantum materials. Here, we introduce a simple model of three antiferromagnetically coupled Kondo impurities, a Kondo triangle'', which can be used to further extend the application of these concepts to electronic systems. We show that by tuning the magnetic frustration, the Kondo triangle undergoes a quantum phase transition between two phases of unbroken symmetry, signaling a phase transition beyond the Landau paradigm. We demonstrate that the frustrated spin liquid'' phase is described by a three-channel Kondo fixed point and thus displays an irrational ground state degeneracy. Using an Abrikosov pseudofermion representation this quantum state is categorized by an emergent U(1) gauge field and its projective symmetry group. The gauge theory is deconfining in the sense that a miniature Wilson loop orders and that topological defects (instantons in the gauge field) are expelled. This phase persists in presence of moderate Kondo screening until the proliferation of topological defects leads to a quantum phase transition to an unfrustrated Fermi liquid phase. Based on this evidence, we propose that the three-channel Kondo phase displays topological order in a similar sense as gapless spin liquids.
Concepts of topological order are central to our understanding of strongly correlated quantum materials. In this work, the authors propose that such concepts -- which transcend the Landau paradigm -- also appear in frustrated impurity models. In particular, they show that a variety of defining characteristics of topological order appear in a frustrated Kondo triangle: an irrational ground-state degeneracy reminiscent of anyons, ordering of a Wilson-loop-like order parameter, and a phase transition driven by the proliferation of monopoles of an emergent gauge field.
There has been significant recent interest in the potential exotic properties of multichannel Kondo lattices and their connection to quantum materials, but the strongly coupled nature of these models has posed challenges to theoretical investigations. We study the emergence of topological order in a two-dimensional two-channel Kondo lattice (2CKL) in a time-reversal symmetry-broken channel symmetric phase using a coupled-wire construction from one-dimensional (1D) 2CKLs. Through bosonization and symmetry analysis, we show that away from particle-hole symmetry, the 1D 2CKL model flows to an infrared fixed point with an emergent $\mathrm{SO}(3,1)\ifmmode\times\else\texttimes\fi{}\mathrm{SU}(2)$ symmetry and a Z2{\mathbb{Z}}_{2}Z2​ gauge structure. Interwire couplings induce a transition to a topologically ordered metallic state characterized by fractionalized excitations and eightfold ground state degeneracy on a torus. Our findings connect impurity-based non-Fermi-liquid behavior to lattice topological order, with potential implications for quantum materials and topological quantum computation.
Kitaev's honeycomb-lattice compass model describes a spin liquid with emergent fractionalized excitations. Here, we study the physics of isolated magnetic impurities coupled to the Kitaev spin-liquid host. We reformulate this Kondo-type problem in terms of a many-state quantum impurity coupled to a multichannel bath of Majorana fermions and present the numerically exact solution using Wilson's numerical renormalization group technique. Quantum phase transitions occur as a function of Kondo coupling and locally applied field. At zero field, the impurity moment is partially screened only when it binds an emergent gauge flux, while otherwise it becomes free at low temperatures. We show how Majorana degrees of freedom determine the fixed-point properties, make contact with Kondo screening in pseudogap Fermi systems, and discuss effects away from the dilute limit.
We investigate quantum impurity problems, where a local magnetic moment is coupled to the spin density of a bosonic environment, leading to bosonic versions of the standard Kondo and Anderson impurity models. In a physical situation, these bosonic environments can correspond either to deconfined spinons in certain classes of Z2{Z}_{2}Z2​ frustrated antiferromagnets, or to particles in a multicomponent Bose gase (in which case the spin degree of freedom is attributed to hyperfine levels). Using renormalization group techniques, we establish that our impurity models, which feature an exchange interaction analogous to Kondo impurities in Fermi liquids, allow flow toward a stable strong-coupling state. Since the low-energy bosons exist around a single point in momentum space, and there is no Fermi surface, an impurity quantum phase transition occurs at intermediate coupling, separating screened and unscreened phases. This behavior is qualitatively different from previously studied spin-isotropic variants of the spin-boson model, which display stable intermediate-coupling fixed points and no screening.
A multichannel Kondo model, where two or more equivalent but independent channels of electrons compete to screen a spin-12\frac{1}{2}21​ impurity, shows overcompensation of the impurity spin, leading to the non-Fermi-liquid behavior in various thermodynamic and transport properties. However, when the channel symmetry is broken, an impurity quantum phase transition can occur at zero temperature. Identification of an order parameter describing the impurity quantum phase transition is very difficult since it is beyond the conventional Landau-Ginzburg-Wilson theory. By employing the natural orbitals renormalization group method, we study both two-channel and three-channel Kondo models, from the perspective of spin correlation between the impurity and electrons in electronic channels. Here we demonstrate that by introducing the spin-correlation ratio as an order parameter we can characterize impurity quantum phase transitions driven by channel asymmetry. In particular, the universal critical exponents $\ensuremath{\beta}$ of the spin-correlation ratio and $\ensuremath{\nu}$ of the correlation length are explicitly determined by finite-size-scaling analysis, namely, $\ensuremath{\beta}=0.10(1)$ and $\ensuremath{\nu}=2.0(1)$, and $\ensuremath{\beta}=0.10(1)$ and $\ensuremath{\nu}=2.5(1)$ for the two-channel and three-channel Kondo models, respectively.
We discuss the two-channel Kondo problem with a pseudogap density of states $\ensuremath{\rho}(\ensuremath{\omega})\ensuremath{\propto}{|\ensuremath{\omega}|}^{r}$ of the bath fermions. Combining both analytical and numerical renormalization group techniques, we characterize the impurity phases and quantum phase transitions of the relevant Kondo and Anderson models. The line of stable points, corresponding to the overscreened non-Fermi-liquid behavior of the metallic r=0r=0r=0 case, is replaced by a stable particle-hole-symmetric intermediate-coupling fixed point for $0<r<{r}_{\mathrm{max}}\ensuremath{\approx}0.23$. For r>rmaxr>{r}_{\mathrm{max}}r>rmax​, this non-Fermi-liquid phase disappears, and instead a critical fixed point with an emergent spin-channel symmetry appears, controlling the quantum phase transition between two phases with stable spin and channel moments, respectively. We propose low-energy field theories to describe the quantum phase transitions, all being formulated in fermionic variables. We employ $\ensuremath{\epsilon}$ expansion techniques to calculate critical properties near the critical dimensions r=0r=0r=0 and 1, the latter being potentially relevant for two-channel Kondo impurities in neutral graphene. We find the analytical results to be in excellent agreement with those obtained from applying Wilson's numerical renormalization group technique.